A mass spectrometer is used to determine the composition of a sample and involves measuring the mass-to-charge ratios and quantities of ions within the sample. One type of mass spectrometer is a tandem or MS/MS mass spectrometer, which has two or more mass analyzers that are arranged in series along an ion path and work in stages. The tandem mass spectrometer also includes ion optics for focusing and propelling the ions along the ion path and between the ion source and each of the mass analyzers.
In a typical tandem mass spectrometer, for example, a sample of material is ionized to form precursor ions. The ions travel into a first mass analyzer that preselects precursor ions having mass-to-charge ratios within a certain range. The precursor ions are then fragmented into product ions. The product ions pass through ion optics that focus and shapes the ion stream so that it conforms to the size and shape of the entrance for the second mass analyzer. The product ions are detected by a detector in the second mass analyzer. The detector outputs a signal embodying information about the ions that it detects.
A problem is that the ions traveling along the ion path tend to repel each other and spread out or diverge from the ion path. Additionally, the ion optics may not precisely shape the ion beam to conform to the shape of the slit. As a result, many of the product ions in the ion stream strike the electrode plate and do not pass through the entrance slit. The transmission efficiency of product ions through the entrance slit of the second mass analyzer can be as low as 5% to 25%, which results in the detector in the second mass spectrometer outputting an information signal having a relatively low amplitude.
Another difficulty relates to noise. In mass spectrometers, both background ions and the ions of interest for analysis may reach the detector. The background ions that reach the detector cause chemical noise that makes it more difficult to pick out and identify the ions of interest. Tandem mass spectrometers improve the filtering of background ions and particles and have a low level of chemical noise, but this improved filtering and ion selection also results in fewer ions reaching the detector. As a result, the amplitude of the information signal output by the detector in the second mass analyzer is further reduced. The problem is that the detector in the second mass analyzer also outputs electrical noise, which is an electrical signal other than the information signal. Noise is a particular problem because the amplitude of the signal output by the detector is proportional to the number of ions striking it. When so few ions reach the detector, it outputs a low signal and the ratio between the signal and the noise (S/N ratio) is very low. The signal can be in effect drowned out by the noise and is more difficult to process.
Additionally, it is necessary to tune and calibrate the mass analyzers. However, the detection circuits for each of the mass spectrometers in a tandem mass spectrometer may not be mismatched (a continuous detection for the quad vs. a pulsed detector for the TOF) with one another. An example is a tandem mass spectrometer in which the first mass analyzer is a scanning quadrupole mass spectrometer and the second mass analyzer is a pulsing time-of-flight mass spectrometer. Mismatched detection schemes can make calibration of the first mass analyzer time consuming, difficult, and even misleading.